1. Technical Field
The present invention relates, generally, to methods and apparatus for time management of manufacturing machinery to improve productivity and, more particularly, to methods and means for time modulation of a multi-mandrel programmable turret so that the mandrels are serially placed in operative association with various work stations positioned about the orbit of the mandrels. A specific implementation of the present invention concerns methods and apparatus for a cup making machine, wherein a turret is rotated at a constant angular velocity while workpieces engaged by the mandrels experience intermittent periods of acceleration, deceleration, and absolute dwell with respect to the work stations in a controlled manner vastly improving throughput while reducing machine fatigue.
2. Description of the Background Art and Technical Problems
Machines comprising intermittently rotatable turrets arranged to convey workpieces to positions or stations where operations are performed by or on the workpieces are generally known. See, e.g., Dearsley U.S. Pat. No. 2,512,922, issued Jun. 27, 1950. Of particular relevance to a preferred implementation of the present invention is the use of indexing turrets in paper cup making machines, for example, as embodied in the PMC-1000 High Production Automatic Paper Cup and Paperboard Can Machine, made by the Paper Machinery Corporation of Milwaukee, Wis.
The PMC-1000 comprises a transfer turret, a mandrel turret and a rimming turret, all synchronously driven in a step-wise manner ("indexed") by a single main drive shaft rotating at a constant angular velocity. Each turret has a number of orbital elements, e.g. mandrels, projecting radially outward from an upright rotatable post. By its stepwise rotation, the mandrel turret carries each mandrel, seriatum, to each of a number of work stations positioned about the mandrel orbit for performing a sequence of predetermined manufacturing steps on a workpiece carried by each mandrel.
Each work station is also operatively associated with a main drive shaft, resulting in synchronous interaction among the mandrels and work stations. Typically, one drive shaft revolution constitutes one machine cycle, during which each work station performs a particular task on the workpiece associated with a given mandrel. During continuous operation of a paper cup making machine, each cup must engage each work station once, the number of work stations on a particular machine being a function of the complexity of the finished cup. Hence, one cup is produced per machine cycle. Conventional multiple turret machines produce between 60 and 200 cups per minute (CPM).
The mandrel turret on the PMC-1000 has eight mandrels equally spaced about its vertical standard. Positioned about the common orbit of the mandrels are six to eight work stations for performing discrete tasks on the workpiece (e.g., paper) carried by each mandrel.
At the first station, the bottom portion of the cup is placed on the outward facing flat end of the mandrel and retained by, for example, a vacuum line communicating with the distal end of the mandrel. Commensurate with the next revolution of the drive shaft, the entire mandrel turret is rotated 45 degrees (360 degrees divided by eight mandrels) so that each mandrel simultaneously moves into engagement with the next work station.
A device called a reformer contacts the circular paper bottom at the second station and bends the periphery thereof slightly away from the not-yet-assembled drinking surface of the cup. The reformed bottom is then transferred to the third station where a pre-heated paper sidewall is folded and clamped around the mandrel and paper bottom. The bottom is heated at the fourth station.
"In-curl" is performed at the fifth station and involves curling the sidewall over the pre-folded portion of a bottom blank. At the sixth station, in the referenced machine a knurling station, the sidewall and bottom are sealed together and squared. At the seventh station, the workpiece is transferred from the mandrel turret to the rimming turret for further processing of the workpiece. After transferring a workpiece to the rimming turret at the seventh station, the mandrel advances to the first mandrel turret station to receive another bottom piece. When the rimming turret operations are completed, the finished cup is exhausted from the machine and stacked for packaging.
Many variations of the above-described process are routinely employed to satisfy unique requirements of different cup design such as, for example, placing a plastic collar on the rim of each cup. Because of space the rim of each cup. Because of space and other mechanical constraints, a maximum of 10 mandrels or work stations may be associated with each turret. Otherwise, the work stations become too crowded, resulting in interference between moving components. Similar limitations impact the ability to adapt current turret designs regardless of the manufacturing process to be performed or the product to be manufactured; these constraints are not unique to cup making.
To ensure that each work station engages and performs its specified task on each workpiece at the appropriate time, the myriad of mechanical apparatus and the turrets with which they cooperate are typically driven by a common main drive shaft. This arrangement leads to heretofore unappreciated implications in certain machine designs.
Horsepower is transmitted from the drive shaft at various points along its length by, for example, belts, pulleys, chains, or gears which supply power to, inter alia, the mandrel turret, rimming turret, blank (sidewall) feeder, sidewall grippers, sidewall folding wings, paper clamp, seam clamp, bottom maker, bottom reformer, bottom heat gun, bottom in-curl, bottom finish, tamper, and rimmer. As many of these mechanisms rotate, index, extend and retract, they bleed horsepower from the drive shaft during some portion of each machine cycle. As a consequence of mechanical inertia these same components often supply horsepower to the drive shaft during other portions of each cycle. Inasmuch as the drive shaft is both constrained to rotate at a constant angular velocity (1-4 revolutions per second) and experiences loading and unloading at various points along its length, the cyclic torques supplied by and imparted to the drive shaft become significant. The mandrel transfer, and rimming turrets, for example, require a combined peak torque of approximately 950 ft.-lbs. to advance from one station to the next. Approximately 400 ft.-lbs. (peak) of torque is supplied by and imparted to the drive shaft by the mandrel turret alone.
The fact that different components interact with the drive shaft at various points along its length results in two interrelated phenomena.
First, while certain apparatus bleed horsepower from the drive shaft at a particular point in each cycle, other components supply horsepower to the shaft at the same point in the cycle. This results in a degree of cancellation of the torque effects upon the drive shaft; the net effect being that the instantaneous torque supplied or absorbed by the drive shaft is less than the sum of the absolute magnitudes of the various torque-absorbing and torque-supplying elements. For a typical machine operating at 200 CPM (200 cups per minute equals 31/3 revolutions per second in a 1:1 machine, as is the exemplary one described above), peak net torque values on the drive shaft range between 775-800 ft.-lbs. supplied and between 700-725 ft.-lbs. absorbed per revolution.
The second phenomenon occurs when the drive shaft simultaneously supplies torques of different magnitudes to a plurality of components coupled to the drive shaft at different points along the length thereof. The application of torques having different magnitudes to differential cross sectional elements of the drive shaft causes "winding" of the drive shaft. Winding can create large, cyclic, torsional stresses and vibrations within the drive shaft.
The combination of high differential torques and the cyclic nature of the loading is capable of producing tremendous cyclic strains in the drive shaft. To prevent material failure, a drive shaft must be of sufficient strength and cross sectional area to effectively distribute the internal loads. Likewise, drive shaft speeds should not exceed a design maximum if undesirably excessive torques are to be prevented.
On the other hand, because the cup-forming operations described above are synchronized with respect to the drive shaft, cup production is a function of shaft speed. The design goal, dictated by commercial practicality and highlighted by the significant investment represented in such a machine, is to maximize the number of cups produced per unit time. Since one cup is produced per drive shaft revolution, the design goal is to maximize drive shaft speed. The difficulty encountered, however, is the fact that torque is a function of the square of the shaft speed. Thus, for a given machine, twice the amount of torque is necessary to yield 282 CPM as is required to yield 200 CPM.
Increasing cup production requires an analysis of drive shaft torque. From first principles, torque is equal to the vector product of force times lever arm. The lever arm is the distance from the axis of the drive shaft to the point at which the drive shaft interacts with the component with respect to which force is being supplied or delivered. For purposes of this analysis, the turrets (mandrel, rimming, and transfer, where applicable), because of their necessarily large mass, impact most significantly on drive shaft torque. Thus, the point on the drive shaft which interacts with a turret is of primary concern.
As discussed above, conventional indexing turrets cooperate with the drive shaft attended with great force. To maintain mechanical precision, the drive shaft gears ("discs") which drive the turrets have relatively large masses. As a result, the distance "r" (from the axis of the drive shaft to the point at which a disc interacts with a turret) for a turret-driving disc is largely dictated by design constraints for given turret forces and drive shaft material properties. The focus, then, becomes the force component of drive shaft/turret torque.
Force is equal to mass times acceleration. In effect, the turret force acting on the drive shaft is a function of the mass distribution of the turret and the angular acceleration imparted to it. Mechanical considerations, particularly strength and vibration characteristics, dictate or are significantly influencial factors respecting the mass of a given turret. Therefore, drive shaft torque can be reduced to the extent turret acceleration can be reduced.
Acceleration is the time derivative of velocity, or the rate at which the velocity (in this case angular velocity) of the turret increases or decreases. In a conventional indexing turret, the mandrels maintain a fixed position with respect to each other and with respect to the turret. As such, the entire turret must be accelerated and decelerated each time the mandrels advance to the next work station. This is a consequence of the fact that precision interaction between workpieces and work stations often requires absolute registration therebetween, i.e., absolute mandrel dwell--otherwise, production quality tends to suffer. As drive shaft speed increases, the torque required to accelerate and decelerate the mass of the turret increases.
It has been suggested by Hoerauf, a German machine manufacturing concern located in the Federal Republic of Germany, that higher drive shaft speeds and higher turret speeds may be achieved if the drive shaft is not required to accelerate and decelerate the turret. Because acceleration is the rate of change of velocity, a turret rotating at a constant velocity, regardless of the magnitude of that velocity, requires no torque (except that required to overcome frictional forces). The heretofore intractable problem with this approach involves establishing absolute registration between the mandrel (carried by a constantly rotating turret) and a work station.
Hoerauf has further suggested a constantly rotating turret for transferring a workpiece from one turret to another. However, either the absolute dwell between the mandrel and the work station must be compromised, or the work station must trace a constant-radius arc about the axis of rotation of the mandrel during the period of engagement. The former is unacceptable for many precision operations; the latter involves extensive additional mechanical complexity and significantly increased space requirements. Thus, neither approach is a satisfactory solution to the general problem, and neither reflects an understanding or appreciation of the underlying problem or its causes.
Others have suggested, albeit in radically different contexts, disposing a plurality of workpieces about the periphery of a rotating wheel. A work station, positioned proximate the orbit of the workpieces, interacts with the workpieces as the wheel rotates.
U.K. Patent No. 2,127,766, published Apr. 18, 1984 and entitled "An Apparatus For Wrapping Sweets", discloses a conveyor wheel mounted on a central drive shaft, having radially extending arms with holding jaws for receiving a workpiece. A jaw retrieves a workpiece at a work station and carries it ninety degrees to a second station where the workpiece is transferred to a perpendicularly disposed pair of jaws. The action of the conveyor wheel is such that, during the reception of the workpiece from the first work station as well as during the delivery thereof to the second work station, the holding jaws are said to observe a "stand-still" relative to the moving conveyor wheel. The apparent dwell of the holding jaws is brought about through the action of a roller lever, which rolls in a cam track disposed about the central axis of the conveyor wheel, itself rotating an adjusting shaft in the opposite sense, thereby causing a so-called "retrogressive coaxial swinging out of the holding jaws". Although the jaws are temporarily biased so that they do not rotate about the axis of the wheel, a necessary consequence is that the workpiece moves radially outward during the period of angular dwell.
Dunn U.S. Pat. No. 2,468,255, issued Apr. 26, 1949 and entitled "Feed Device", discloses a feed turret for transferring a workpiece to a main turret. The feed turret is provided with up to six or more sets of lever mechanisms which are hingedly connected to and disposed symmetrically about the main axis of the feed turret. The distal end of each lever system comprises an object-carrying means. The main turret likewise includes a plurality of object-carrying devices, reciprocable in the main turret member and symmetrically disposed about the central axis thereof. The feed turret transfers an object to the main turret as the two corresponding turret devices pass each other, or experience "transferring registration," which extends for approximately 25 degrees of travel. In addition, a dwell zone is provided during a period in which an operator loads the workpiece onto the feeder turret. Through the interaction of two stationary cams and a series of levers and followers, transferring registration and dwell are effected without interrupting turret rotation. However, because the lever mechanisms do not pivot about the axis of rotation of the turret, each object-carrying means rotates about its own axis during dwell. Thus, a condition of absolute dwell, essential to many precision operations, cannot be obtained via the teachings of the '255 patent.
U.K. Patent No. 2,069,440, published Aug. 26, 1981 and entitled "Improvement in Wrapping Machines," discloses a transfer wheel which rotates about a shaft within a frame. A cam track is rigidly secured to the frame. A plurality of arms are pivotally mounted on a spindle secured to the transfer wheel. As the wheel rotates, the arms, biased by the cam track, can accelerate or decelerate with respect to the transfer wheel or bunch up or space apart with respect to each other, as desired. However, as the arms accelerate or decelerate, their distances from the axis of the transfer wheel necessarily increases or decreases.
Similarly, Zambomi U.S. Pat. No 4,511,027, issued Apr. 16, 1985 and entitled "Method and Apparatus for the Handling of Products by Operative Means Carried in Continuous Movement," discloses a pair of continuously rotating spoked wheels, there being blocks slidably mounted on the spokes. The wheels are aligned so that a workpiece carried by a block on one wheel may be transferred to the corresponding block of the mating wheel as the blocks undergo transferring registration. Although the blocks experience relative dwell with respect to each other, they do not experience dwell with respect to their own hub or a fixed point in space.
All of these devices have several common features. For example, the rotating wheels are disposed to interact with one or two work stations. For a workpiece which must interact with at least six to eight work stations, this would require a plurality of mandrel turrets. In addition, a relatively large period of time is required to move a workpiece into engagement with a work station, as compared to the period of time a workpiece experiences engagement with a work station.
To the extent that the goal of increasing cup production depends on increasing shaft speed, the manner of eliminating or compensating for the increased torques heretofore associated therewith has eluded the industry. An awareness of the previously unappreciated subtle effects of increased torques and an understanding of the advantages to be derived from absolute mandrel dwell are critical to the implementation of cost effective production enhancement techniques.